Overwatch isn't really that much of a deterrent from being assaulted, in my experiences. The best way I've seen and used to limit Overwatch is when assaulting a unit, I try my best to have 2 units assaulting at the same time. Send the "weaker" of the 2 units first, and let Overwatch fire be shot at them, and then have the "stronger" unit charge afterwards, avoiding Overwatch completely.
Also, think twice before assaulting Snipers or units comprised of numerous Characters. While playing against a Dark Eldar player, he tried to assault a unit of my Snipers with his Wyches. On my Overwatch, I rolled 6 6's, wounding on 4 of those rolls. He lost his Succubi and both Hydra Gauntlets as a result of that (+the one regular Wych). Granted, I got ridiculously lucky rolling that many 6's on only 10 shots, but the end result of that Assault Phase obviously worked out in my favor.